U.S. Pat. No. 4,159,365 discloses copolyesters of phenylhydroquinone and terephthalic acid containing up to 10 mole percent (based on the total moles of units) of other aromatic or cycloaliphatic polyester-forming units, including hydroquinone. Ten mole percent of the "other aromatic polyester-forming units" above, if an aromatic diol, corresponds to 20 mole percent based on the total moles of aromatic diols combined. In column 2, lines 29-31, the above patent teaches: "The presence of comonomer units depresses the melting point as would be expected." The copolyesters of this invention prepared from terephthalic acid, 50 to 75 mole percent of a diacyl ester of phenylhydroquinone, and 25 to 50 mole percent of a diacyl ester of hydroquinone melt similar to or higher than the homopolymer prepared from terephthalic acid and a diacyl ester of phenylhydroquinone. In addition copolyesters of this invention have unexpected chemical resistance.
U.S. Pat. No. 3,160,602 discloses a process for the preparation of aromatic polyesters from dihydric phenols and aromatic carbonyl halides while dissolved in certain solvents. The use of phenylhydroquinone is disclosed (column 6, line 12), but there is no disclosure concerning which of the numerous copolyesters can be prepared which are sufficiently soluble in the solvents for useful molecular weights to be attained.
U.S. Pat. No. 4,153,779 discloses somewhat similar copolyesters prepared with substituted phenylhydroquinones. The halogen and alkyl substituents reduce the stability of those polymers in an oven at 260.degree. C., however, whereas the copolyesters of this invention have superior stability (films remain tough appreciably longer when heated in an oven at 260.degree. C.).